Silver-containing direct thermographic imaging materials are non-photosensitive materials that are used in a recording process wherein images are generated by the direct application of thermal energy and in the absence of a processing solvent. These materials have been known in the art for many years and generally comprise a support having disposed thereon one or more imaging layers comprising (a) a relatively or completely non-photosensitive source of reducible silver ions, (b) a reducing agent composition (acting as a black-and-white silver developer) for the reducible silver ions, and (c) a suitable hydrophilic or hydrophobic binder. Thermographic materials are sometimes called “direct thermal” materials in the art because they are directly imaged by a source of thermal energy without any transfer of the energy or image to another material (such as in thermal dye transfer).
In a typical thermographic construction, the image-forming thermo-graphic layers are based on silver salts of long chain fatty acids. The preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms, such as behenic acid or mixtures of acids of similar molecular weight. At elevated temperatures, the silver(I) of the silver carboxylate is reduced by a reducing agent (that is, the developer) whereby a black-and-white image of elemental silver is formed.
Problem to be Solved
As noted above, direct thermographic materials are imaged by a recording process wherein images are generated by imagewise heating a recording material containing chemical components that provide an optical density change in an imagewise fashion. The chemical components include a reducing agent as noted above. Many compounds have been described in the art that are considered useful for this purpose. They are often compounds having at least two adjacent hydroxy substituents as described, for example, in U.S. Pat. No. 5,582,953 (Uyttendaele et al.) and U.S. Pat. No. 6,093,528 (Terrell et al.).
The phenolic reducing agents of the prior art have varying usefulness, and vary in their light stability, resistance to aerial oxidation, tint and/or tone of the developed silver image, and their ability to produce a dense black metallic silver image under the short time and high temperature conditions that occur during thermal printing.
A particular problem with thermographic materials is their change in tint and tone upon storage. The initial blue-black image typically becomes more red-black in color upon storage. Thus, a challenge in designing thermo-graphic materials is the need to improve the “Dark Stability” of the imaged and processed thermographic film upon storage. It is desirable that the Dmin not increase, and that the Dmax, tint, and tone of the image not change.
Thus, there remains a continuing need to provide improved black-and-white imaging chemistry including reducing agents for direct thermographic materials that generate a dense neutral and storage stable black metallic silver image upon thermal imaging.